The fossilized eyes of 54-million-year-old crane-flies offer the first known record of a melanin light-screening pigment in any arthropod. This discovery is reported in Nature this week. Additional analyses reveal how structures of the crane-fly eye are altered by the fossilization process, which indicates that previous interpretations of the eye structures of other fossilized arthropods, such as trilobites, may need to be reassessed.
Compound eyes, found in arthropods such as insects and crustaceans, are the most common visual organs seen in the animal kingdom. These eyes have an evolutionary history that dates back at least 520 million years; studying early examples can offer insights into the visual capacity of arthropod eyes.
Johan Lindgren and colleagues study the composition and microanatomy of 54-million-year-old crane-fly eyes from Denmark, and compare them with those of modern crane flies to determine how the fossilization process alters compound eyes. They observe evidence of eumelanin - a pigment that could serve as a shield to protect photoreceptors from light - in both the fossilized and modern specimens.
The fossilized eyes show signs of calcification on the lens structures, which the authors suggest replaced chitin, a material that protects the eye and is involved in vision. Previous studies have suggested that, in some extinct arthropods, this calcification may have occurred in life, but the authors propose that such mineralization may have impaired vision. Instead, they propose that calcium deposits are an artefact of the preservation of fossils. This finding implies that long-standing hypotheses of calcified eyes in trilobites, one of the earliest known groups of arthropods, may require reconsideration.